1. Field of the Invention
The present invention relates to the measurement of certain physical properties of fluids and, more particularly, to the determination of the thermal conductivity and specific heat of fluids.
2. Description of the Prior Art
A number of approaches have been devised to measure the thermal conductivity, thermal diffusivity and specific heat of a fluid of interest. A traditional approach to determining thermal diffusivity, D.sub.t, and specific heat, c.sub.p, has been via calorimetry using reversible step increases of energy fed to a thermally isolated or adiabatic system. Such devices are bulky, slow and cumbersome.
Approaches for determining the thermal conductivity, k, of fluids typically use various types of detectors including resistive bridge type sensors. One such approach is described in U.S. Pat. No. 4,735,082 in which thermal conductivity is detected using a Wheatstone bridge in which a filament in one leg of the bridge is placed or positioned in a cavity through which the sample gas of interest is passed. The filament is used to introduce a series of amounts of thermal energy into the fluid of interest at various levels by varying the input voltage which, are, in turn, detected at another leg as voltage difference signals. Integration of the changes of the value of the successive stream of signals yields a signal indicative of the heat dissipation through the fluid, and thus, the thermal conductivity of the fluid.
Further to the measurement of thermally induced changes in electrical resistance, as will be discussed in greater detail below, especially with reference to prior art FIGS. 1-5, very small and very accurate "microbridge" semiconductor chip sensors have been described in which etched semiconductor "microbridges" are used as heaters and sensors. Such sensors might include, for example, a pair of thin film sensors around a thin film heater for measuring flow rates. Semiconductor chip sensors of the class described are treated in a more detailed manner in one or more of patents such as U.S. Pat. Nos. 4,478,076, 4,478,077, 4,501,144, 4,651,564, and 4,683,159, all of common assignee with the present invention.
An improvement for measuring the thermal conductivity, thermal diffusivity and specific heat of a fluid is disclosed in U.S. Pat. No. 4,944,035 to Aagard et al. Aagard et al. discloses using a microbridge structure that has a heater film and at least one spaced sensor films. A pulse of electrical energy is applied to the heater at a level and duration such that both a transient change and a substantially steady-state temperature occur in the sensor. The thermal conductivity of the fluid of interest is determined based upon a known relation between the sensor output and the thermal conductivity at steady-state sensor temperatures. The specific heat and thermal diffusivity of the fluid of interest are determined based on a known relation among the thermal conductivity, the rate of change of the sensor output during a transient temperature change in the sensor, and the thermal diffusivity and specific heat.
A limitation of the Aagard et al. approach is that the relation among the thermal conductivity, the rate of change of the sensor output during a transient temperature change in the sensor, and the thermal diffusivity and specific heat must be previously determined and stored in a data bank. The measured rate of change at the sensor output must then be correlated to the desired specific heat value using the previously determined relation. This may require a significant amount of support hardware and/or software.
Another limitation of Aagard et al. is that the derived specific heat value is dependent on the rate of change of the sensor output between two reference temperatures. That is, the rate of change of the sensor output may depend on the amplitude of the sensor output signal, and thus the resistance of the sensor. It is known that the resistance of many materials vary with time, at least to some degree, thus adding an additional potential error source. It would be desirable, therefore, to provide a more direct approach to determine the thermal conductivity, thermal diffusivity and specific heat of a fluid of interest, independently of the amplitude of the sensor output.